from datetime import datetime
import logging
from decimal import Decimal
from typing import Any, Dict, List, Optional, Union
from accounting.models import (
    Account,
    AccountType,
    EntryType,
    JournalEntry,
    Transaction,
    TransactionStatus,
)
from sqlalchemy import func
from sqlalchemy.orm import Session
from core.accounting_validator import validate_double_entry, DoubleEntryValidationError
from core.decimal_utils import to_decimal

logger = logging.getLogger(__name__)

class LedgerError(Exception):
    """Base class for ledger exceptions"""
    pass

class UnbalancedTransactionError(LedgerError):
    """Raised when debits and credits do not match"""
    pass

class EventSourcedLedger:
    """
    Service for recording immutable financial events.
    Ensures every transaction follows double-entry principles.
    """

    def __init__(self, db: Session):
        self.db = db

    def record_transaction(
        self,
        workspace_id: str,
        transaction_date: datetime,
        description: str,
        entries: List[Dict[str, Any]],
        source: str = "manual",
        external_id: Optional[str] = None,
        metadata: Optional[Dict[str, Any]] = None
    ) -> Transaction:
        """
        Record a double-entry transaction.
        'entries' should be a list of dicts: [
            {"account_id": "...", "type": EntryType.DEBIT, "amount": Decimal("100.00")},
            {"account_id": "...", "type": EntryType.CREDIT, "amount": Decimal("100.00")}
        ]
        """
        # 1. Validate balance using exact Decimal comparison (NO EPSILON)
        try:
            validation = validate_double_entry(entries)
            # If we get here, transaction is balanced
        except DoubleEntryValidationError as e:
            # Re-raise as UnbalancedTransactionError for compatibility
            raise UnbalancedTransactionError(
                f"Debits ({e.debits}) do not match Credits ({e.credits}). "
                f"Difference: {e.difference}"
            ) from e

        # 2. Create Transaction Header
        transaction = Transaction(
            workspace_id=workspace_id,
            transaction_date=transaction_date,
            description=description,
            source=source,
            external_id=external_id,
            status=TransactionStatus.POSTED,
            metadata_json=metadata
        )
        self.db.add(transaction)
        self.db.flush() # Get transaction ID

        # 3. Create Journal Entries
        for entry_data in entries:
            journal_entry = JournalEntry(
                transaction_id=transaction.id,
                account_id=entry_data["account_id"],
                type=entry_data["type"],
                amount=entry_data["amount"],
                description=entry_data.get("description")
            )
            self.db.add(journal_entry)

        try:
            self.db.commit()
            logger.info(f"Recorded transaction {transaction.id} for workspace {workspace_id}")
            return transaction
        except Exception as e:
            self.db.rollback()
            logger.error(f"Failed to record transaction: {e}")
            raise LedgerError(f"Database error: {str(e)}")

    def get_account_balance(self, account_id: str) -> Decimal:
        """
        Calculate the current balance of an account.
        Asset/Expense: Debit - Credit
        Liability/Equity/Revenue: Credit - Debit
        """
        account = self.db.query(Account).filter(Account.id == account_id).first()
        if not account:
            return Decimal('0.00')

        # Sum debits and credits
        totals = self.db.query(
            JournalEntry.type,
            func.sum(JournalEntry.amount).label("total")
        ).filter(JournalEntry.account_id == account_id).group_by(JournalEntry.type).all()

        debit_total = Decimal('0.00')
        credit_total = Decimal('0.00')
        for t in totals:
            amount = Decimal(str(t.total)) if t.total else Decimal('0.00')
            if t.type == EntryType.DEBIT:
                debit_total = amount
            else:
                credit_total = amount

        # Assets and Expenses are typically debit accounts
        if account.type in [AccountType.ASSET, AccountType.EXPENSE]:
            return debit_total - credit_total
        else:
            # Liabilities, Equities, and Revenues are typically credit accounts
            return credit_total - debit_total

    def get_trial_balance(self, workspace_id: str) -> Dict[str, Decimal]:
        """Returns the balances of all accounts in the workspace"""
        accounts = self.db.query(Account).filter(Account.workspace_id == workspace_id).all()
        return {acc.name: self.get_account_balance(acc.id) for acc in accounts}

class DoubleEntryEngine:
    """Helper for common accounting patterns"""
    
    @staticmethod
    def create_payment_entry(
        cash_account_id: str,
        expense_account_id: str,
        amount: Union[Decimal, str, float],
        description: str
    ) -> List[Dict[str, Any]]:
        """Pattern: Pay for an expense with cash"""
        decimal_amount = to_decimal(amount) if not isinstance(amount, Decimal) else amount
        return [
            {"account_id": expense_account_id, "type": EntryType.DEBIT, "amount": decimal_amount},
            {"account_id": cash_account_id, "type": EntryType.CREDIT, "amount": decimal_amount}
        ]

    @staticmethod
    def create_invoice_entry(
        receivable_account_id: str,
        revenue_account_id: str,
        amount: Union[Decimal, str, float],
        description: str
    ) -> List[Dict[str, Any]]:
        """Pattern: Issue an invoice (Revenue earned, but not yet received)"""
        decimal_amount = to_decimal(amount) if not isinstance(amount, Decimal) else amount
        return [
            {"account_id": receivable_account_id, "type": EntryType.DEBIT, "amount": decimal_amount},
            {"account_id": revenue_account_id, "type": EntryType.CREDIT, "amount": decimal_amount}
        ]

    @staticmethod
    def create_bill_entry(
        payable_account_id: str,
        expense_account_id: str,
        amount: Union[Decimal, str, float],
        description: str
    ) -> List[Dict[str, Any]]:
        """Pattern: Receive a bill (Expense incurred, but not yet paid)"""
        decimal_amount = to_decimal(amount) if not isinstance(amount, Decimal) else amount
        return [
            {"account_id": expense_account_id, "type": EntryType.DEBIT, "amount": decimal_amount, "description": description},
            {"account_id": payable_account_id, "type": EntryType.CREDIT, "amount": decimal_amount, "description": description}
        ]

    @staticmethod
    def create_payment_for_bill(
        cash_account_id: str,
        payable_account_id: str,
        amount: Union[Decimal, str, float],
        description: str
    ) -> List[Dict[str, Any]]:
        """Pattern: Pay off a recorded bill"""
        decimal_amount = to_decimal(amount) if not isinstance(amount, Decimal) else amount
        return [
            {"account_id": payable_account_id, "type": EntryType.DEBIT, "amount": decimal_amount, "description": description},
            {"account_id": cash_account_id, "type": EntryType.CREDIT, "amount": decimal_amount, "description": description}
        ]